There are two typical radiation sources used for optical pumping of a magnetometer helium isotope resonance cell. Conventional helium isotope magnetometers use a helium lamp as a source of optical radiation for optically pumping the resonance cell and for optically observing the state of the helium isotope sample in the resonance cell. A second, more recent approach is the proposed use of a solid state laser radiation source.
A helium lamp when excited by electrical energy emits infrared radiation which is used for optical pumping and for monitoring of the helium isotope in the resonance cell. The radiation emitted by the helium lamp consists of three spectral lines at the approximate wavelength of 1083 nm which are designated respectively D.sub.0, D.sub.1, D.sub.2. These spectral lines derive from transitions between the triplet P energy levels and the triplet S energy level. The 2S.sub.1 to 2P.sub.0, 2S.sub.1 to 2P.sub.1, and 2S.sub.1 to 2P.sub.2 transitions have center wavelengths of 1082.91 nm, 1083.01 nm and 1083.03 nm, respectively.
There are a number of disadvantages associated with helium lamp pumping. One disadvantage is that the achievable radiation intensity is less than the level required for applications where improved magnetometer sensitivity is desired. Another disadvantage is that optical pumping with a helium lamp is inefficient. The lamp emits all three spectral lines at 1083 nm, two of which pump counter the third, thereby reducing the achievable sensitivity of the helium magnetometer. An additional problem with helium lamp pumping is that the helium lamp is inefficient in conversion of electrical energy into pumping radiation at 1083 nm. A significant part of the electrical energy required to excite the lamp is wasted on heating the glass lamp walls and simultaneous excitation of a variety of unwanted and undesirable helium spectral lines. A still further disadvantage of helium lamp pumping is degraded magnetometer accuracy caused by the lamp radiation. Because the frequencies of the helium lamp radiation are not matched precisely to each corresponding absorption center frequency of the helium atoms in the resonance cell, the accuracy of the helium magnetometer is degraded since the pumping radiation appears to the helium isotope sample as the presence of an additional unwanted magnetic field in addition to the magnetic field to be measured.
It has been recognized that a single line laser radiation source could overcome the disadvantages of helium lamp pumping. However, during the first 25 years following the invention of the laser no suitable laser has been developed for this purpose. In recent years, the solid state laser has emerged as a radiation source. While the solid state laser does provide single line pumping, there are a number of disadvantages associated with its use in helium optical magnetometers. The list of disadvantages include the necessity for a relatively rare and expensive special-purpose neodymnium dopped laser crystal. Another disadvantage is that the laser crystal must be pumped by a high-power diode laser. Further, the solid state laser must be frequency stabilized by mechanically adjusting optical elements or in some cases the cavity length. To accommodate the laser crystal, the diode laser pump, and the frequency stabilization and tuning functions, the solid state laser cavities are of the order of ten centimeters in length. Yet another disadvantage is that additional expensive optical elements must be added to couple the radiation from the pump diode laser into the crystal and to couple the radiation from the laser source into an optical fiber which conducts the laser radiation to the helium isotope cell. Finally the extremely small alignment tolerances for all optical elements of the solid state laser result in a laser that is very difficult to mechanically package for the rough mechanical and thermal environment encountered by a portable helium magnetometer.
A need has thus arisen for a compact and efficient source of laser radiation for optical magnetometers which use a helium isotope as the resonance element. A need has further arisen for a laser radiation source which provides single mode, frequency stabilized radiation for optical magnetometers.